1. Field of the Invention
The present invention relates to semiconductor single-crystal wafers that can be used as substrates of various electronic devices, and more particularly to alleviation of cracking and improvement in cost effectiveness in utilizing compound semiconductor single-crystal wafers, which are relatively liable to crack.
2. Background Art
Semiconductor single-crystal wafers, when used as substrates for semiconductor electronic devices, must as a matter of course be impervious to cracking during the process of manufacturing the semiconductor electronic devices. The reason is that a semiconductor single-crystal wafer that has cracked in the course of a process cannot be put through subsequent processing, meaning that the wafer goes to waste.
In addition to silicon single-crystal wafers, wafers of single-crystal semiconductor compounds have been utilized in recent years as substrates to produce various electronic devices. Among such compound semiconductor single crystals, hexagonal AlxGayIn1−(x+y)N (0<x≦1, 0≦y<1, x+y≦1) single crystal is a preferable candidate material for manufacturing various electronic devices. It should be noted that in the present specification, “AlxGayIn1−(x+y)N (0<x≦1, 0≦y<1, x+y≦1) semiconductor” will also be referred to as AlGaInN semiconductor for short.
As mentioned in the Japanese Journal of Applied Physics, Vol. 40, 2001, pp. L426–L427, AlGaInN single crystal has a lower fracture toughness than silicon single crystal and therefore tends to be liable to crack. In particular, AlN substrates are liable to crack during handling since they have a low fracture toughness, on the order of a fraction of that of SiC substrates and sapphire substrates.
Growing an AlGaInN single crystal of large diametric span has conventionally been difficult in comparison with growing a silicon single crystal, and an AlN single crystal of large diametric span has been particularly difficult to grow. In recent years, however, growing a single crystal of AlN into a relatively large diametric span is becoming feasible.
When the principal faces of a semiconductor wafer have a small area, the weight of the wafer is accordingly small and shock acting on the wafer during processing will be small, and thus likelihood that the wafer will crack during processing is low. For this reason, when a semiconductor wafer has a small diameter, the thickness of the wafer is determined based on requirements for easier wafer handling. For example, too thin a wafer thickness causes difficulty in picking up the wafer with pincers, so the thickness of the wafer is set to be such a thickness that the wafer can be easily picked up with pincers.
In contrast, when a wafer has a large diametric span, the weight accordingly increases and shock acting on the wafer also tends to be large; therefore, the requirements arising from the perspective of making the wafer impervious to cracking become more important than the requirements necessary for easy wafer handling. In other words, it is desirable that as the area of the principal faces of a wafer becomes larger, the thickness of the wafer be made greater in order to make the wafer impervious to cracking.
Nevertheless, although increasing the thickness of the wafer makes cracking less likely to occur even with wafers of large diametric span, the efficiency in utilizing the semiconductor single-crystal as substrates for producing electronic devices drops, which is undesirable in terms of cost effectiveness.